1. Field of the Invention
The present invention relates to a method of disposing selectively two types of substances on the surface of a substrate.
2. Description of Related Art
In recent years, intensive research and development efforts have been made in various sectors to fabricate electronic devices, such as thin-film transistors, light-emitting display elements, and microlenses, by a printing method instead of a conventional photolithography method. The reason why a printing method has attracted attention as a method of fabricating electronic devices is as follows. Since the components of an electronic device can be formed directly on a substrate if a printing method is used, both the number of fabrication steps and the number of materials to be used can be reduced compared to when the photolithography method is used. As a result, the cost of manufacturing electronic devices may be reduced significantly.
The size of a thin-film transistor, a light-emitting element, or a microlens generally is several tens to 100 μm. Therefore, the formation of such an electronic device by a printing method requires a printer capable of forming a pattern of several tens to 100 μm in size. Among various types of printers, an ink jet printer and a gravure offset printer can form a pattern as small as several tens μm in size, and there has been reported methods of fabricating electronic devices using these printers.
JP 3541625 B2 (Document 1) and JP 3858809 B2 (Document 2) each disclose a method of fabricating a light-emitting element for an organic electroluminescence display by an ink jet printing method. JP 2006-318850 A (Document 3) discloses a method of fabricating a light-emitting element for an organic electroluminescence display by a gravure offset printing method.
As another printing method, there has been proposed a method in which hydrophilic regions surrounded by a water-repellent region are formed on a substrate in advance, and an ink is applied to the substrate by a spin coating method or a dipping method, or using a brush, so as to dispose the ink selectively onto the hydrophilic regions. Hereinafter, this method is referred to as a dewetting method.
In recent years, the dewetting method has attracted attention as a method of fabricating electronic devices. The principle of the dewetting method is described briefly below, and then conventional examples using this method are shown.
The basic procedure of the dewetting method includes a step of bringing an ink into contact with a substrate, and a step of separating the ink from the substrate.
FIG. 10A to FIG. 10E are schematic cross-sectional views showing how the ink is disposed in a hydrophilic region in this basic procedure.
FIG. 10A is a schematic cross-sectional view showing a movement of a droplet of an ink 122 that has been brought into contact with a substrate 121 in air 130. The droplet of the ink 122 moves in the direction of an arrow 123. According to the movement of the droplet, the ink 122 is brought into contact with the surface of the substrate 121 and then separated therefrom. In FIG. 10A, according to the movement of the ink 122, a common interface 128 at which three interfaces among the ink 122, the substrate 121 and the air 130 contact with each other moves in the direction of the arrow 123.
FIG. 10B to FIG. 10E are enlarged views of the vicinity 124 (see FIG. 10A) of the common interface 128. FIG. 10B shows the beginning of the movement of the ink 122. At this point in time, the common interface 128 is located on the water-repellent region 125. Then, as shown in FIG. 10C, the common interface 128 moves on the water-repellent region 125 as the ink 122 moves, and reaches the boundary between the hydrophilic region 126 and the water-repellent region 125.
As the ink 122 further moves, the center of the gravity of the entire ink 122 also moves in the direction of the arrow 123 as shown in FIG. 10D, but the common interface 128 stays on the boundary between the water-repellent region 125 and the hydrophilic region 126.
As shown in FIG. 10B and FIG. 10C, the common interface 128 on the water-repellent region 125 moves according to the movement of the ink 122. On the other hand, as shown in FIG. 10D, the common interface 128 on the boundary between the hydrophilic region 126 and the water-repellent region 125 stays there. Presumably, this is because the adhesion that acts between the ink 122 and the water-repellent region 125 is weaker than the adhesion that acts between the ink 122 and the hydrophilic region 126.
As used herein this specification, the phrase “the adhesion that acts between a liquid (ink in the above case) and a water-repellent region is weaker than the adhesion that acts between the liquid and a hydrophilic region” means that the amount of work required to separate the liquid that has adhered to a unit area of the water-repellent region is smaller than the amount of work required to separate the liquid that has adhered to a unit area of the hydrophilic region.
Next, as shown in FIG. 10E, as the ink 122 further moves, a part of the ink 122 is cut and disposed in the hydrophilic region 126. Subsequently, the common interface 128 is located on the water-repellent region 125 adjacent to the hydrophilic region 126 in the direction of the movement 123 of the ink 122.
When the spacing between the adjacent two hydrophilic regions 126 and 129 is small, the common interface 128 may move to the next hydrophilic region 129, in some cases, upon cutting the ink 122 on the hydrophilic region 126.
In the example shown in FIG. 10A to FIG. 10E, the ink is brought into contact with the substrate and separated therefrom according to the movement of the ink droplet on the substrate.
Such contact and separation of the ink also can be performed by a spin coating method, a dipping method, or a brush method.
The spin coating method is a technique of applying an ink to a substrate by dropping the ink on the entire surface of the substrate and then rotating the substrate at high speed. In this method, dropping of the ink brings the ink into contact with the substrate, while a centrifugal force generated by the rotation of the substrate separates the ink from the substrate.
The dipping method is a technique of applying an ink to a substrate by dipping the substrate into the ink and then pulling up the substrate out of the ink. In this method, dipping of the substrate brings the ink into contact with the substrate, while pulling-up of the substrate separates the ink from the substrate.
The brush method is a technique of applying an ink to a substrate by moving a brush impregnated with the ink on the substrate. In this method, moving of the brush both brings the ink into contact with the substrate and separates the ink from the substrate.
JP 2007-129227 A (Document 4) and “S. P. Li et al.; Applied Physics Letters 89, 122105-1 to 3 (2006)” (Document 5) each disclose a method of fabricating a field-effect transistor using the dewetting method. In this method, hydrophilic regions corresponding to the shape of source electrodes and drain electrodes as well as a water-repellent region surrounding these hydrophilic regions are first formed on the surface of a conductive silicon substrate having a surface coated with a 100 nm thick thermal oxidized silicon film.
In this method, the hydrophilic regions and the water-repellent region are formed in the following manner.
First, a mold with a relief pattern, in which the shape of recesses correspond to the shape of source electrodes and drain electrodes, is produced using a polydimethylsiloxane (PDMS) material.
Next, this relief mold is impregnated with a solution of 1H,1H,2H,2H-perfluorododecyltrichlorosilane (CF3(CF2)7C2H4SiCl3), and then the mold is pressed against a substrate. The CF3(CF2)7C2H4SiCl3 impregnated into the projections is transferred onto the substrate to form a water-repellent thin film.
As a result, hydrophilic regions having the shape of electrodes and water-repellent region that surrounds the hydrophilic regions are formed on the substrate.
Subsequently, an ink in which a conductive polymer material is dissolved is applied to the substrate by the brush method, so that the conductive polymer is disposed only in the hydrophilic regions.
The resulting pattern of the conductive polymer corresponds to the source electrodes and the drain electrodes.
Next, an ink in which a polymeric organic semiconductor is dissolved is applied to the substrate by a spin coating method. As a result, a bottom-gate field-effect transistor is fabricated.
JP 2007-500587 T (Document 6) discloses a method of printing on a flexible substrate by a continuous roll-to-roll process using the dewetting method. In one example of this method, using a flexographic printing method, a fluororesin pattern is formed on the surface of a polyethylene terephthalate resin substrate, which has been undercoated with gelatin. Then, aqueous glycerol, conductive polymer, liquid crystal, or the like is formed by printing in hydrophilic regions by a hand coating method.
“Daniel M. Hartmann, Osman Kibar, and Sadik C. Esener; Optics Letters, 2000, vol. 25, pp. 975-977” (Document 7) discloses a method of fabricating a microlens by the dewetting method. In this method, a thin film pattern made of water-repellent wax is first formed on a substrate by the photolithography method, so that circular hydrophilic regions surrounded by a water-repellent region are formed.
Next, an ultraviolet curable resin is applied to this substrate by the dipping method, and then the resin is irradiated with ultraviolet rays to be cured. In this manner, microlenses with a diameter of 50 μm are formed in the hydrophilic regions.
“Fang-Chung Chen et al.; IEEE Photonics Technology Letters, 2006, 18, pp. 2454-2456,” (Document 8) also discloses a method of fabricating a microlens by the dewetting method. In this method, circular hydrophilic regions surrounded by a water-repellent region are formed on a substrate by the same method as used in Document 7, an ultraviolet curable resin is applied to the substrate by the spin coating method, and then the resin is irradiated with ultraviolet rays. Thus, microlenses are fabricated.
Applicators used in the dewetting method are simple in structure and inexpensive compared with ink-jet printers and gravure offset printers. Therefore, the use of the dewetting method may enable the fabrication of electronic devices at lower cost.
Due to the processing limitation of an ink jet head and a mold, it is considered to be difficult to reduce the size of the minimum pattern to be printed by the ink jet method and the gravure offset method to less than 10 μm.
In contrast, in the dewetting method, the size of hydrophilic regions can be reduced to 1 μm or less by using a PDMS mold and the photolithography method. Therefore, the use of the dewetting method enables the printing of a pattern of 1 μm or less.
As described above, the dewetting method has attracted attention because it has a potential to print micro-patterns at low cost.
It is, however, difficult to print selectively two types of inks (two types of substances) in predetermined regions on the same substrate by the dewetting method. This is because in the dewetting method, two types of regions, that is, a water-repellent region and a hydrophilic region are formed on a substrate and a difference between the adhesion that acts between the ink and the water-repellent region and the adhesion that acts between the ink and the hydrophilic region is utilized to dispose the ink thereon.